In the Mix

Nov. 26, 2007

The Sedalia Wastewater Treatment Plant, located in Sedalia, Mo., utilizes a MixAir treatment system, designed by Aqua-Aerobic Systems, Inc., to process its wastewater. The technology was chosen for its ability to efficiently treat wastewater at design values of biological oxygen demand (BOD) and total suspended solids (TSS), with the added ability to reduce electrical consumption during periods when the system is underloaded organically, without experiencing solids deposition.

During operation of the plant, Sedalia operators have seen the low effluent BOD and TSS values they expected, and they are able to save thousands of dollars per year in electrical consumption due to benefits derived from this flexible system design.

Treatment requirements

Sedalia's MixAir system consists of one basin designed to treat an average flow of 2.5 million gal per day (mgd) and a peak flow of 7.2 mgd. The system basin is 130 ft in diameter, with a water level of 17 ft.
The influent for the system was designed for a BOD of 200 mg/L, TSS of 200 mg/L and ammonia of 36 mg/L. Actual influent data showed an average flow of 1.4 mgd, with influent BOD of 171 mg/L, TSS of 197 mg/L and ammonia of 22.5 mg/L. Effluent testing showed a BOD of <1 mg/L, TSS of 6 mg/L and ammonia of <1 mg/L.

The MixAir system was designed for an actual oxygen requirement (AOR) of 9,290 lb of oxygen per day, based on supplying 1.4 lb O2/lb BOD applied and 4.6 lb O2/lb NH3 applied as specified. Based on the influent test data, the system is operating at an average AOR of 4,003 lb of oxygen per day, or 43% of the design AOR.

"The system has consistently provided higher-quality effluent with BOD averaging around 5 mg/L and TSS averaging around 2 mg/L. Overall, it runs well, and Aqua-Aerobic has provided fine customer service when we needed it," said Larry Mundy of the Sedalia Wastewater Treatment Plant.

Description of operation

The Sedalia plant requires a minimum airflow of 3,185 standard cu ft per minute (SCFM) from the blowers to keep the tank contents in suspension. The aeration system was designed to deliver approximately 3,403 SCFM to meet the oxygen requirements at design load.

At first glance, this would lead one to believe that the system is oxygen limited, not mixing limited; however, the current organic load is only 43% of the design load. The aeration system has not needed to operate with the blowers on a continuous basis to meet the oxygen requirements. The mixer completely mixes the aeration basin and operates continuously in the aeration basin in order to maximize the amount of contact between the influent and the biomass.

If the mixer were not installed in the tank, the aeration system would need to operate at 3,185 SCFM in order to keep the tank contents in suspension and prevent the biomass from settling in the tank, even though the current oxygen demand is much lower.

Aerating the basins only on an as-needed basis has saved Sedalia thousands of dollars in electrical consumption. Over time, electrical consumption as a function of design load based on utilization of the MixAir concept has saved the Sedalia facility approximately $70,000 per year by only operating the blowers as needed to meet the oxygen requirements.

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